U.S. Pat. No. 5,748,371, issued May 5, 1998 and entitled xe2x80x9cExtended Depth of Field Optical Systems,xe2x80x9d is incorporated herein by reference.
1. Field of the Invention
This invention relates to apparatus and methods for increasing the depth of field and decreasing the wavelength sensitivity of incoherent optical systems. In particular, this invention relates to extended depth of focus optics for human vision.
2. Description of the Prior Art
In the human eye, it is well known that the accommodation of the lens decreases with age, and the bifocal or trifocal glasses are eventually needed in many cases. When a human lens must be replaced, an intraocular implant is usually designed for viewing objects at infinity, and the person then uses reading glasses and other glasses of various strengths for vision at closer distances.
Current techniques that are used experimentally in intraocular implants provide two or more foci, for reading and distance vision, for example. This is done either with a shorter focal length lens placed in the center of a lens of a longer focal length, for example, or by use of diffractive optics that provides two foci. The result is one in-focus image and one out-of-focus image. The human brain disregards the out-of-focus image and concentrates on the in-focus image. The major disadvantage of this technique is that if the two images are not aligned (as occurs when the lens is not centered, a frequent occurrence with contact lenses) the images do not line up and the out-of-focus image is apparent. As such a two-foci contact lens moves, the images move with respect to each other. Another disadvantage is loss of contrast. That is, the image looks washed out. The situation is even worse when the object is located between a reading distance and a very long distance; examples include the distance to a computer screen, a television set, or music on a stand. In these cases, two poorly focussed images are superimposed.
Another commonly used approach is called monovision: a person is fitted with a lens on one eye for reading, and another lens on the other eye for distance viewing. The brain then selects the best focussed image to concentrate on. Again, images of objects that are at an intermediate distance cannot be seen clearly. Otherwise, this approach works for many people, but the inability to fuse images that are not both focussed has made this solution unusable for many others. In that case, the user sees two misregistered images.
The human brain can adapt to unchanging visual conditions, even when they markedly affect the immediate visual perception. An example of this was discussed above, where the brain is able to adapt to two images if one is in focus, by concentrating on the in-focus image and ignoring the other.
As another example, the human brain can accommodate for the very large distortions present in varifocal lenses, which gradually move from providing clear vision at a distance, for objects seen through the upper portion of the lens, to providing clear vision of close objects when seen through the lower inside part of the lenses. Objects at an intermediate distance can be seen through the center of the lenses.
An extreme example of how the brain can adapt to unchanging conditions was shown in experiments where mirrors were used to invert the images seen by a person. After a day or so, the brain turned the images upside down, so that the person saw a normal image.
The human brain cannot adjust to conventional out-of-focus images, because the amount of blur changes with misfocus, or with distance from the in-focus plane. In addition, the blur is such that some information about the object being seen is lost.
There is a need to extend the depth of focus of the human eye by modifying contact lenses, intraocular implants, and the surface of the eye itself (with laser surgery, for example).
An object of the present invention is to provide extended depth of focus (EDF) by modifying contact lenses, intraocular implants, and natural human eyes. This is accomplished by applying selected phase variations to the optical elements in question (for example, by varying surface thickness). These phase variations EDF-code the wavefront and cause the optical transfer function to remain essentially constant within a large range away from the in-focus position. The human brain undoes the EDF-coding effects, resulting in an in-focus image over an increased depth of focus. While the human brain cannot compensate for general out-of-focus images, where the amount of blur changes with distance from the in-focus plane, it can compensate for the specific EDF-coding misfocus added by the optical mask, because that misfocus does not change with distance, and the phase variations are selected so that no information is lost in the process.
For cases where the person still has some refocussing capability, the eye will change focus such that the image of the object being viewed falls into the extended region where the brain can decode the image. In the case of an, intraocular implant to replace a damaged lens, the amount of wavefront coding is tailored to give the required amount of invariance in the point spread function. The depth of focus can be increased to be 800% or greater than that of the normal eye.
The selected phase variations to be applied to the optical element (for example, by varying surface thickness) are asymmetric phase distributions that modify the point spread function of the imaging system so that it does not change over a very large distance. There are a variety of wavefront coding shapes that can be used, including cubic phase functions.